The invention relates to a rock drilling machine comprising at least: a frame; a percussion element for generating stress pulses; a shank arranged at the front of the percussion element in the percussion direction, the shank comprising a percussion surface for receiving said stress pulses; and further an axial bearing comprising at least: a first piston and a second piston; between the pistons, an axial first contact surface and an axial second contact surface, the contact surfaces being located in the same pressure space; at least one pressure duct for leading pressure fluid from a pressure source to the axial bearing; pressure surfaces in the pistons, on which surfaces the pressure fluid is arranged to act for axial movement of the pistons; and in which axial bearing the pistons are arranged to push the shank along a different travel length towards the percussion direction; the force of said pistons, by the action of the pressure fluid towards the percussion direction, being dimensioned such that the percussion surface is adjustable during drilling at the desired axial point for receiving the stress pulses.
The invention further relates to an axial bearing for a percussion rock drilling machine, the axial bearing comprising at least: a frame; at least a first piston and a second piston arranged in a space formed in the frame, both comprising at least one pressure surface; at least one pressure duct for leading pressure fluid to said pressure surfaces for axial movement of the pistons; and, between the pistons, axial contact surfaces located in the same pressure space.
It is known to use an axial bearing in a rock drilling machine for moving the shank to the intended percussion point during drilling and for adjusting the percussion power by adjusting the position of the shank. On the other hand, the axial bearing can be used to dampen stress pulses reflected from the rock back to the drilling machine. The axial bearing disclosed in U.S. Pat. No. 6,186,246 comprises two nested sleeves arranged in a space around the percussion piston, and of which one is in indirect contact with the rear end of the shank. The travel lengths of the sleeves towards the percussion direction are different and the sleeves are in contact with one another via an axial contact surface. A separate pressure duct for leading pressure medium is connected to a working pressure surface at the rear end of both sleeves. The outer sleeve is sealed to the frame and to the inner sleeve, and, furthermore, the inner sleeve is sealed to the percussion piston and to the outer sleeve. During drilling, both sleeves are able to rotate around their shafts and, furthermore, the sleeves can collide with each other in the axial direction. Accordingly, the contact surfaces between the sleeves are subjected to mechanical stress that wears them. A similar problem may present itself in the solution according to U.S. Pat. No. 5,896,937.